SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



199 



proximately equal branches. The leaves of the fern, unlike those of flower- 

 ing plants, bear reproductive bodies on their lower surfaces in the form of 

 minute "fruit dots," or sort. There are no flowers in the pteridophytes. 

 The spermatophytes, or seed plants, not only are the most highly or- 

 ganized of plants but also are the dominant plants of the modern world. 

 Since we have already examined the structure and functioning of the 

 members of this group in detail, 

 it will be unnecessary to review 

 these matters here. 



Plants That Lack Chlorophyll 



Although plants typically possess 

 chlorophyll and manufacture food 

 by photosynthesis, certain of 

 them — clearly plants, as judged by 

 structure and life history — lack this 

 substance and must obtain their 

 food in other ways. Such plants are 

 encountered only among the mem- 

 bers of the lowest and the highest 

 of the plant divisions — the thallo- 

 phytes and the spermatophytes. 

 Physiologically they fall into two 

 groups — those which (like the green 

 plants) manufacture their own 

 food, but by means of chemical in- 

 stead of solar energy; and those 

 which must like animals obtain 

 their food ready-made. 



The plants which depend upon chemosynthesis instead of photosyn- 

 thesis for food manufacture are few. They include the kinds of bacteria 

 called hydrogen, iron, nitrifying, and sulfur bacteria, which respectively 

 oxidize hydrogen, certain iron compounds, ammonia, and hydrogen 

 sulphide. These oxidative reactions release energy that is used for the 

 synthesis of carbohydrate from carbon dioxide and water. The nitrifying 

 bacteria, aided by the prior activities of other bacteria yet to be discussed 

 which release ammonia in the soil, play an important role in the cycle of 

 nitrogen utilization by plants. They are of two sorts — nitrite bacteria, 

 which convert ammonia into nitrites, and nitrate bacteria, which oxidize 

 the nitrites into nitrates, the nitrogen compounds most readily utilized 

 by green plants. 



Except for these few chemosynthetic bacteria, all other colorless plants 

 fall into the second group — those which must obtain ready-made organic 



La.. 13.15. A tree fern in the northern 

 Philippines. {Photo by Charles Martin (c) 

 NGS. Courtesy National Geographic Society.) 



